Air-water flows at hydraulic structures: Experimental investigations of interfacial characteristics and air-water mass transfer

多相流中的气-水传质是水工结构设计的关键。空气夹带过程通常发生在溢洪道，落井，静力池或液压脉冲涡轮机的外壳中。目前关于加气流动的知识主要依赖于实验模型研究。对于实验室实验，必须考虑到依赖的尺度效应，以确保模型与自然之间的相似性，这意味着几何、运动和动态的相似性。在加气流动中，几何相似模型不能满足模型和原型中弗劳德数、雷诺数和韦伯数的一致值。实验结果表明，即使在1:2或1:3比例模型中，气泡大小和湍流尺度等参数也可能受到比例效应的影响。在原型规模（1:1）的实验几乎还没有。因此，未来的研究必须集中在新的现场测量上，在现场进行全尺寸的测量(Chanson 2013：曝气流动的水力学：qui pro quo？水利学报，51(3),223-243。挥发性气体在液体中的传质主要取决于界面面积、扩散系数和气体浓度梯度。在这种情况下，质量传递方程的积分代表了一种确定质量传递的分析方法。初步调查在昆士兰大学的一个小倾斜的阶梯滑道上进行。直接比较溶解氧测量和质量传递方程的数值积分显示了两种方法之间的良好一致性（Tombees 2005：阶梯水道上的空气-水质量传递，Journal of Environmental Engineering, 1377-1386）。这些有希望的结果，大规模测量的迫切需要，以及加气流动研究的最新进展，如信号处理和不同实验的比较分析，都有理由在这一研究领域开展新的研究。在拟建项目范围内，将在实验室和原型规模上对陡坡斜槽（1V:0.8H）进行详细的多相流和氧合测量。计划中的原位测量将有助于弥补由于缺少大规模调查而产生的现有研究差距，收集的数据将用于量化结垢效应，特别是关于溶解氧和曝气效率。此外，将空气-水流动特性的实验结果用于传质方程的数值积分，并将结果与氧传递速率的测量结果进行比较。通过同时测量氧传递和空气-水流动特性，获得了有关加气流动描述的大量知识。

Air-water mass transfer in multiphase flows plays a key role in designing hydraulic structures. Air entrainment processes typically occur in spillways, drop shafts, stilling basins or in the casing of hydraulic impulse turbines. Current knowledge of aerated flows mainly relies on experimental model investigations. For laboratory experiments, dependent scale effects have to be taken into account by ensuring similarity between model and nature, implying geometrical, kinematic and dynamic similarity. In aerated flows, consistent values of Froude-, Reynolds- and Weber-number in model and prototype cannot be fulfilled in geometrical similar models. Experiments results show that some parameters, such as bubble sizes and turbulent scales, are likely to be affected by scale effects, even in 1:2 or 1:3 scale models. Experiments in prototype scale (1:1) are barely available yet. Consequently, future research has to focus on new field measurements, performed in situ at full-scale (Chanson 2013: Hydraulics of aerated flows: qui pro quo? Journal of Hydraulic Research, 51(3), 223-243). The mass transfer of a volatile gas into a liquid mainly depends on interfacial area, diffusion coefficient and the gradient of the gas concentration. In this context, the integration of the mass transfer equation represents an analytical approach determining mass transfer. Preliminary investigations were conducted on a stepped chute with small inclination at the University of Queensland. Direct comparisons between dissolved oxygen measurements and numerical integration of the mass transfer equation showed good agreement between the two methods (Tombees 2005: Air-water mass transfer on a stepped waterway, Journal of Environmental Engineering, 1377-1386). These promising results, the urgent need of large-scale measurements and recent progresses in researching aerated flows, e.g. in signal processing and comparative analyses of different experiments, give reason to conduct new investigations in this field of research. Within the scope of the proposed project, detailed multiphase flow and oxygenation measurements on a stepped chute with steep inclination (1V:0.8H) will be conducted in laboratory and prototype scale. The planned in-situ measurements will contribute to close the existing research gap arising from missing large-scale investigations and the collected data will be used to quantify scaling effects, especially with respect to dissolved oxygen and aeration efficiency. Furthermore, experimental results of air-water flow properties are used for numerical integration of the mass transfer equation and results will be compared with measurements of the oxygen transfer rate. With the simultaneous measurement of oxygen transfer and air-water flow properties, substantial knowledge concerning the description of aerated flows is gained.